STACKED - Mother's Day Giveaway

For Mother's Day, STACKED are giving away a 3 stack of bangles: 28th Street, 42nd Street, and a Signature Collection bangle of the winner's choice (a $750 retail value)!  Here is the link to enter: www.stackednewyork.com/moms

Just fill out the fields by May 10, 2014 and one winner will be chosen at random and contacted on Mother’s Day via e-mail.

Coatings Used to Protect Silver Alloys

It is important to acknowledge one thing before we start our discussion of the different protective coatings available for use with silver alloys – all silver alloys eventually tarnish. The only way to prevent this is to place a physical barrier between the silver alloy and the atmosphere/sweat or chemicals which cause the tarnish reaction to take place. 

Anti-tarnish coatings can be broadly split into five families: 

a) Metal Coatings – i.e. Rhodium. This is a platinum group metal which is plated over the top of silver jewellery to give a hard, white, un-reactive surface layer. It is typically only a thin coating of 0.2-0.3 microns thick and it will wear off as a jewellery item is worn. The underlying silver alloy is usually a different colour to the rhodium so the worn areas are noticeable; more so as the underlying silver starts to react with the atmosphere. 

b) Passivating Solutions – i.e. Self Assembled Monolayer based on Thiols. The description may seem scary but these are simply the ‘dip’ solutions which offer protection against tarnishing. They contain long chain molecules (thiols) which attach themselves to the surface of the silver alloy and create a chemical surface layer which repels water. For a tarnish reaction to take place there needs to be moisture present on the surface of the silver so the presence of this thiol coating prevents the reaction starting. These coatings are between 0.05 - 0.15 microns thick and although they offer good protection on display items they can be easily rubbed off as a piece is worn. 

c) E-coatings – Acrylic or Polyurethane Electrophoretic Coatings. These are specialist coatings which are applied by electrodeposition and they form a thin clear lacquer on the surface of the piece after they have been cured. As with any plating process careful control of the coating solutions and filtration of the rinse waters is necessary to form a coating which is non-porous. On decorative products the coating thickness is usually between 2-5 microns and although these coatings have reasonable wear characteristics even at the 2-5 microns thickness they can be detected visually and by touch.  

d) Lacquer Coatings - Cellulose Nitrate or Acrylic Coatings. These coatings require a high degree of skill to apply to get an even coating, particularly in the hidden or hard to reach areas. They are typically between 5 -50 microns thick and offer good resistance to tarnish but can degrade and yellow when exposed to ultraviolet light (in display cases the tungsten light bulbs typically used also emit ultraviolet light). They are easily detectable visually and have a ‘plastic’ feel when touched. 

e) Oxide coatings – Atomic Layer Deposition. These coating are mixtures of metal oxides that are applied by either a physical vapour deposition or a chemical vapour deposition process. It is an expensive process to carry out as the machines used to apply these coatings need to be able to produce a very low vacuum and require regular maintenance. The coatings they produce are about 0.8-1 microns thick and have good tarnish resistance; however they are very easily removed when the coated piece is worn. 

So how do the Argentium silver alloys compare to these different barrier coating protection techniques? 

Argentium silvers also work by producing a protective oxide layer at their surface but in this case the layer is not applied but is self generated as the germanium content of the alloy oxidises naturally in air. The protective germanium oxide is slowly worn away as the piece is worn, but this only exposes fresh germanium at the surface of the piece. This germanium then oxidises in air to renew the protective oxide layer. This self-generation of the protective surface layer is a unique characteristic of Argentium silver alloys and while it does not offer complete protection from tarnish it does remarkably slow the rate at which Argentium silver alloys tarnish compared to other silver alloys that are commercially available.

Different Silver Alloys

One thing I am always asked is, “how is Argentium silver different from other silver alloys”? With there being so many different alloys now available I thought I would try to categorise and summarise each alloy type’s particular quirks.

Traditional sterling silver – This is the simple 92,5% silver, 7.5% copper combination. A good basic alloy with good hardness. For silversmiths the problems of firescale and tarnish are well documented.

Spinning silvers – Historically these alloys had part of the copper content of the traditional sterling silver composition replaced with cadmium (usually about 2%). This gave an alloy which was about 10-15HV lower in hardness than traditional sterling silver which had excellent deep drawing and stamping characteristics. Cadmium containing silvers are now prohibited by worldwide legislation and attempts to create similar alloys by simply replacing the cadmium with tin or zinc failed because the oxides of the tin and zinc, formed when torch annealing, were very hard to remove. Some manufacturers now use Britannia silver (95.8% silver, 4.2% copper) as a spinning silver because of its lower hardness.

Deox silver alloys – These are alloys developed primarily for casting applications and their properties were recently reviewed in an excellent paper presented at the 2010 Santa Fe Symposium by Joerg Fischer Buhner (click here to download pdf). Some of the copper content of these alloys is replaced with zinc and/or silicon with the aim of giving bright, firestain free castings. While the silicon and zinc additions do limit the formation of firestain it is not always a complete success. The higher silicon content alloys can be more difficult to cast consistently and cannot be fabricated easily limiting their use; whereas zinc is well documented to fume at typical investment casting temperatures. These alloys are an improvement on the traditional silver composition for investment casters but have no significant benefits for the practicing silversmith working with sheet and wire.

Platinum group metal additions – these are additions of either gold, palladium or platinum which replace some of the copper content of the traditional sterling silver composition. Aside from the considerable cost implication of replacing copper with a precious metal it has yet to be demonstrated that an alloy which contains an addition of one of these elements has a significantly improved tarnish resistance compared to the deox alloys detailed earlier. Gold additions make the alloy more yellow; platinum creates difficulties on melting and increases the potential for hard spots to form and silver alloys containing palladium have shown sensitivity to ultraviolet light when tested under ‘showroom’ conditions.

Alloys containing germanium - the mechanism by which the germanium content of Argentium silver alloys protects against tarnish and firestain by forming a transparent germanium oxide is something that I will discuss in another blog. Our work has shown that the composition of the Argentium silver alloys with about a 1% germanium addition optimises the mechanical working and casting characteristics of the Argentium silver alloys while giving exceptional tarnish resistance and firestain resistance. Other alloys may contain germanium, but these are limited to contents below 0.5% germanium because of our patent protection and they have to rely on supplemental additions of zinc and tin to try to match the performance of Argentium silver alloys.